Harvard: There are lots of what if here. The diagram in your last post although could work, it is probably not the way the Power Company has connected it physically. If you have a voltage drop that can be read with a meter, then you will get current flow through a resistive and/or impedance electrical path.
If an over head bare high voltage power line had broke and is lying on the ground, you can’t assume the earth is a pure conductor (0 resistances to current flow). This is the reason why grounding and bonding methods are so important. The powers that make the Electrical Codes can’t make it 100% safe.
The OP trailer could have a bonding or grounding problem that is causing him to feel a shock. I say “could” because what if the wet ground was providing the 18 volts from the power cord sitting in the water and the chassis of his trailer is a less resistance path to the source’s safety ground, meaning his trailer chassis is wired properly? At some time or another you’re going to read a voltage from chassis to earth ground, the code’s wiring methods are there to hopefully reduce that voltage so the no harm comes to us. From a keyboard, I can’t assume were the OP’s problem is, but he knows he had a problem and is taking steps to correct it.
FYI:
The power company use meters to detect ground fault currents and they are not volt meters from a hardware store. A GFI receptacle doesn’t monitor voltage to ground. 18 volts I wouldn’t think a human would feel a strong tingle, at least I don’t? There is a good reason why the power’s to be picked no more then 120 volts to ground and not a subject that can be taught on a RV Forum.
